1. Field of the Invention
The present invention relates to a radiation measuring apparatus used for emission control or radiation control in a nuclear reactor facility, a spent nuclear fuel reprocessing facility, or the like.
2. Description of the Background Art
A radiation measuring apparatus used in a nuclear reactor facility, a spent nuclear fuel reprocessing facility, or the like is required to perform measurement so as to cover a wide range, from about 10 cpm to about 107 cpm, of the repetitive frequency of a signal pulse inputted as a result of detection of radiation. Therefore, in order to eliminate discontinuity due to range switching, a count rate measurement is performed using an up-down counter that operates at a high speed over a wide range without range switching.
The above measuring method calculates a count rate by a calculation section reading in a constant cycle an addition-subtraction accumulated value when addition input and subtraction input are balanced, and particularly, has a feature of being able to measure over a range up to a high count rate. In addition, as compared to a method of calculating a count rate based on a count value measured by a counter, the above measuring method also has a feature of being able to measure with high resolution and high accuracy over the entire measurement range from a low count rate to a high count rate.
However, the counting method using an up-down counter takes a long time for recovery once noise has intruded, because the indication value returns to its original value through response by a time constant. Therefore, there is a problem that the measurement missing time until the indication has recovered to the normal state needs to be minimized.
In order to solve the problem, the following invention is disclosed. That is, noise intrusion is detected, and the last count rate is held when noise has intruded. When the noise intrusion has stopped, a backup count rate is calculated based on the count value of a counter provided in parallel with an up-down counter and the last count rate just before the entry and the backup count rate is outputted. Then, when the regular count rate based on the up-down counter has recovered to the backup count rate, the output count rate is returned to the regular count rate. Thus, increase in the indication due to noise intrusion is prevented (for example, see Patent Document 1).
In addition, the radiation measuring apparatus used in the above facilities is required to measure with constant accuracy against statistic variation unique to radiation, and to rapidly respond to the increase in the count rate. Therefore, the up-down counter operates such that a standard deviation of the count rate becomes constant over the entire measurement range and that the time constant becomes inversely proportional to the count rate, thereby balancing an addition-subtraction accumulated value. The calculation section calculates the count rate based on the addition-subtraction accumulated value.
In order to confirm that this calculation processing normally functions, the radiation measuring apparatus has a switch and a test pulse generation section. Thus, at one point equal to or higher than a high-alarm point in the entire measurement range, a test pulse having the corresponding repetitive frequency is automatically inputted, thereby enabling soundness confirmation (operation test). However, there is a problem that it takes a long time to recover from a test mode to a measurement mode.
According to this problem, the following invention is disclosed. That is, when the switch is returned from the test mode to the measurement mode, the indication value is automatically made to respond by a fast time constant until the indication value recovers to an allowable range of the stored indication value just before the test. Also in the case where the soundness confirmation is performed at a plurality of alarm set points by connecting a dedicated testing apparatus to the radiation measuring apparatus, the recovery time from the test mode to the measurement mode is reduced in the same manner (for example, see Patent Document 2).
Patent Document 1: Japanese Laid-Open Patent Publication No. 2008-215907 (Paragraphs [0040] to [0047] and FIG. 4)
Patent Document 2: Japanese Laid-Open Patent Publication No. 2009-63351 (Paragraphs [0029] to [0033] and FIG. 2)
In the invention disclosed in Patent Document 1, during recovery time of the addition-subtraction accumulated value of the up-down counter after noise intrusion, a backup count rate is calculated by using a count value of the counter obtained in a constant cycle and the backup count rate is outputted. Then, when the regular count rate based on the addition-subtraction accumulated value of the up-down counter has recovered to the backup count rate, the output count rate is returned from the backup count rate to the regular count rate. However, since the count rate responds exponentially, it takes a long time to recover to the regular count rate. Although this period is not a measurement missing time but measurement can be performed, there is a problem of temporarily losing the original feature of the up-down counter that measurement can be performed with favorable resolution and high accuracy over the entire measurement range from a low count rate to a high count rate.
In addition, in the invention disclosed in Patent Document 2, when the test mode is switched to the measurement mode to recover the indication, the recovery time is reduced by automatically switching the time constant to a fast one. However, there is a similar problem that it takes a long time for recovery because of response in an exponential manner.